The invention relates to an elevator control apparatus and, more particularly, to the one using the thyristor-Leonard system.
With a remarkable progress of semi-conductor devices, the so-called thyristor-Leonard system has been prevailingly used for the control apparatus of an elevator system using a DC motor for the cage drive.
When the thyristor-Leonard system operates in the regenerative braking mode, current flows into the thyristor power rectifier and the AC power source, as is well known. In the regenerative braking mode, if power stoppage occurs, a voltage developed in the armature of the DC motor causes a current flow. At this time, the thyristor power rectifier is made ineffective in its function of control so that the current grows rapidly to be excessive one. This excessive current possibly gives a shock to the passengers in the cage or frequently damages the thyristors in the power rectifier. The conventional system is so designed as to detect the power stoppage and shut off such an excessive current. A relatively long time, e.g. 50 to 100 milli-seconds, is necessary for the shutting-off of the excessive current by the conventional one, however.